Smoke Alarms

[Henryv]

Warm greetings.
I had two smoke alarms fitted when I had the house re-wired a few years ago. They were wired in.
They started to bleep so I knew I had to change the back up batteries.
I switched them off at the mains and removed the batteries but they still bleeped until I put the new batteries in.
How could they do that without power?


Henry.

 

[electronicsuk]

I had some old Aico detectors do the same thing. I assume it's because there's a filtering capacitor somewhere after the AC-DC power supply, and as the detectors use such little current, they'll run a little while on the charge that's left in the cappy.